1. Field of the Invention
The present invention relates to a glass ceramic panel or cook top having a cooking surface provided with a pleasing black decoration by applying a decorative paint to it.
2. Description of the Related Art
Modern cooking ranges have a glass ceramic panel providing a cooking surface, which is partially or also completely coated with decorative paint. These decorative paints are generally divided into the “glazing” or “enamel” category. A “glazing” is generally a glass that is clear or colored in the melt (glass flux), while an “enamel” is a coating material, which contains a colored non-transparent material, such as a pigment, in the glass flux. Colored inorganic compounds can be used as pigments. However the pigments should usually not react or only react slightly with the glass flux.
The decorative paint is burned in at temperatures, which are below the softening point of the glass ceramic. In this process the respective glass compositions of the glazing or the enamel are melted and form a stable bond with the surface of the glass ceramic.
The applied decorative paints have different functions. They serve especially for marking cooking zones, for characterizing touch-panels and for decorative design of the cooking surface. The decorative design has the purpose, among others, of protecting the underlying glass ceramic from usage marks (flecks and scratches).
Particularly popular decorative paints are gray, white and black. The manufacture of a decorative paint, which produces a deep black color on a glass ceramic panel, is very difficult. Instead a dark brown or dark gray color impression is usually produced, when a commercial black pigment and the usual glass flux is used in the decorative paint, because of a chemical reaction between pigment surfaces and glass fluxes during the burning in or baking process. This is caused by the presence of zinc oxide in the glass flux, which shifts the color shade toward brown tones, so that the burned in decorative paint is dark brown or dark gray—but not black. However glass fluxes with a zinc oxide content of up to 4 percent by weight are used in decoration of glass ceramic cooking surfaces, for example GF1 as described in DE 197 21 737 C1, GF2 as described in DE 198 34 801 C2 or GF3 as described in EP 460 863 B1. The reason is that the presence of zinc considerably improves the viscosity behavior during burning in and increases the abrasion resistance of the decorative paint.
Currently no deep black colors can be produced with zinc-containing glass fluxes in spite of careful selection of black pigments.
An additional serious disadvantage of commercial “black” decorative paints is that they are substantially rougher than other decorative paints. Thus e.g. the Ra values according to DIN EN ISO 4288 of the currently used “black” decorative paints (“black 1” and “black 2”) are considerably greater than white or gray decorative paints. Since cooking surfaces coated with rough decorative paints very easily collect dirt (e.g. by abrasion of fine metal parts of pans or vessel bottoms) and the rubbed-off particles are especially noticeable, high quality decorative paints for cooking surfaces—especially black decorative paints—should be very smooth (Ra≦0.35μ).
The color impressions produced during burn-in using conventional “black” decorative paints are listed in Table I. The color value determination was based on the L*a*b* CIELAB color system. The roughness values of these applied known white, gray and black decorative paints are listed in Table I, which appears hereinbelow.
TABLE ICONVENTIONAL GRAY AND BROWN ENAMELS MADE FROMDIFFERENT BLACK PIGMENTS AND THEIR COLOR VALUESExamplePigment*Glass FluxColorL*a*b*1Black 2700GF1Dark Brown250.2−0.42Black 2700GF2Dark Gray250.2−1.43Black 2700GF3Gray280.2−1.84Black 240137GF1Dark Gray240.2−0.65Black 240137GF2Dark Gray250.2−1.76Black 240137GF3Gray280.2−2.07Black PS3000GF1Dark Brown250.2−0.68Black PS3000GF2Dark Gray250.1−1.69Black PS3000GF3Dark Gray270.1−1.8*The source of Black 2700 is Johnson Matthey, the source of Black 240137 is Degussa and the source of Black PS3000 is Reimbold & Strick.L* a* and b* are color value parameters in the CIELAB system.
The pigment fraction in all examples of decorative paints listed in Table I was 10 percent by weight (i.e. 90% glass flux) and the coating thickness was from 3 to 4μ. The color measurement was performed with a Mercury 2000 spectrophotometer, Series 1165, of Datacolor (light type, D65, Observation angle: 10°). The glass flux compositions were taken from DE 197 21 737 C1 (GF1), DE 198 34 801 C2 (GF2) and EP 460 863 B1 (GF3).